1. Field of the Invention
The present invention relates to an element substrate on which a plurality of printing elements are arranged while being divided into groups each including a predetermined number of printing elements, print data is supplied to each group, and adjacent printing elements in each group are not simultaneously driven in printing. The present invention also relates to a printhead and a printing apparatus having the element substrate.
2. Description of the Related Art
A printhead, which discharges ink droplets in a direction perpendicular to a surface and has heaters for generating a thermal energy, is known as one of ink jet printheads which use heat as an energy for ink droplet discharge. In a printhead of this type, generally, ink to be discharged is supplied from the reverse side of the element substrate having the heaters via ink supply ports extending through the element substrate.
U.S. Pat. No. 5,173,717 discloses a printhead having a plurality of printing elements arrayed in a line. In a printhead of this type, the plurality of printing elements is divided into blocks. Several driving integrated circuits are arranged on a single element substrate to simultaneously drive the printing elements in each block. Image data is arranged in correspondence with each printing element, thereby executing arbitrary printing on a print medium.
Such printhead widely uses a driving method of dividing printing elements into a plurality of blocks, as described above, and sequentially driving the blocks for the purpose of, for example, reducing the maximum necessary power for driving the printing elements. U.S. Pat. No. 5,357,268 is known as a prior-art technique of divisional block driving.
Particularly, when one printing element is continuously driven, the print density may change due to accumulated heat. The printing element is also affected by the heat of the heater of an adjacent printing element.
When adjacent printing elements are driven simultaneously, the pressure generated upon ink discharge causes crosstalk between the fluid channels of the printing elements. This crosstalk may change the print density. Hence, after driving the printing elements, a quiescent time is preferably inserted to avoid the crosstalk.
As a technique of preventing the above-described problems, distributed driving is known, which distributes simultaneously drivable printing elements in the direction of a printing element array. According to the distributed driving, adjacent printing elements are never driven simultaneously. It is therefore possible to eliminate the influence of adjacent printing elements by inserting a quiescent time.
On the other hand, there has been provided a method of obtaining high image quality, in which the amount of discharge per dot is decreased by reducing the size of a heater included in each printing element. That is, the image quality is improved by reducing the dot size. To increase the print speed, the driving frequency is raised by driving while using a pulse which is shorter than before. However, to drive a smaller size heater having for higher image quality at a higher frequency, as described above, the sheet resistance value must be large.
The relationships between various driving conditions for different heater sizes as shown in FIG. 13A will briefly be described. FIG. 13B shows the changes in the sheet resistance value (Ω/□) and the current value (A) with respect to the driving pulse width (μS) in a heater having a large size (A) and that having a small size (B). FIG. 13C shows the relationships between the sheet resistance value (Ω/□), the current value (A), and the driving voltage (V) in the heater having a large size (A) and that having a small size (B).
As is apparent from the relationships between the driving conditions and the heater sizes in FIGS. 13B and 13C, to drive the small heater under the same conditions as those for the large heater, the sheet resistance value needs to be large. When the heater is driven under a large sheet resistance value and a high driving voltage, the consumed current value becomes small. Since the energy consumption in the resistor portion except the heater decreases, energy saving can be achieved. This effect is particularly large in a printhead including a plurality of heaters.
U.S. Pat. No. 6,769,762 discloses a heater formed from a thin film of TaxSiyNz (the ratio of the numbers of atoms is x:y:z=20 to 80:3 to 25:10 to 60). This arrangement implements a high-resistance heater characteristic capable of coping with a smaller dot size and enables energy saving in a printhead.
The heater used in a printhead must be able to increase the resistance and maintain desired performance. More specifically, the heater in a printhead raises the temperature to 600° C. to 700° C. upon receiving short pulses, generates bubbles in ink, and discharges it. The high temperature state and the room temperature state are repeated at a high frequency. For this reason, if the heater cannot maintain its performance, the resistance value of the heater may change and pose problems in ink discharge.
More specifically, a printhead generally performs constant voltage driving. Hence, when the resistance value decreases, the current that flows to the heater increases, and the overcurrent extremely shortens the life of the heater. To the contrary, when the resistance value increases, the current decreases, and ink discharge may become impossible. Even after the above-described history of use, the resistance value variation of the heater must be at a minimum.
Such a change in the heater performance can be predicted to some extent by evaluating the temperature coefficient of resistance (TCR characteristic) of the material of the heater. As is known, generally, the smaller the TCR characteristic is (zero ideally), the better a heater can maintain its performance. In developing a heater material, it is very important to simultaneously satisfy the high resistance and the performance maintaining. U.S. Pat. No. 6,769,762 describes that a preferable TCR characteristic can be obtained at a resistivity of 2,500 μΩ·cm or less. U.S. Pat. Nos. 4,392,992, 4,510,178, and 4,591,821 disclose CrSiN films as a material for obtaining a high sheet resistance.
Recent techniques of increasing the printed image quality tend to aim at eliminating graininess in effect. For this purpose, the amount of discharge of a droplet is preferably 1 pl or less.
To cause a number of printing elements to discharge ink in an amount of discharge of 1 pl or less at a high driving frequency, it is necessary to stabilize discharge by suppressing temperature rise without lowering the driving voltage. For example, when the driving voltage is 24 V, the pulse width is 1 μs, and the heater size is 17 μm×17 μm, the sheet resistance must be 700Ω/□ or more.
In the above-described TaSiN, a preferable TCR characteristic is obtained at a resistivity of 2,500 μΩ·cm or less, as described in U.S. Pat. No. 6,769,762. That is, to achieve the recently required sheet resistance of 700Ω/□ or more (resistivity of 3,000 μΩ·cm or more) in the above-described TaSiN, the TCR characteristic degrades, and the performance cannot be maintained. When the resistance is raised to maintain the performance, a problem of productivity such as a large resistivity variation rises. It is therefore necessary to find a new material which simultaneously satisfies the higher resistance and the performance maintaining. From the viewpoint of productivity as well, a new material that ensures a sufficient margin to maintain the performance against the variation in the resistivity is required.
U.S. Pat. Nos. 4,392,992, 4,510,178, and 4,591,821 disclose CrSiN films as a material for obtaining a high sheet resistance as described above. However, in these CrSiN films, when a voltage having a pulse width in actual printing is applied about 1.0×104 (1.E+04) times, the resistance value changes from the initial resistance value, as shown in FIG. 9. In this state, excess power may be applied to the heater in printing, and the heater may break.
For a thermal printer which performs divisional block driving, a thermal ink jet head having a heater heating mode using a lower applied voltage than that in printing to stabilize the driving of heaters is known. In the heater heating mode of Japanese Patent Laid-Open No. H5-31899, all heaters are simultaneously driven at a voltage lower than that in printing, thereby stabilizing the driving of the heaters.
A plurality of print chips, each of which has heaters and is used in an ink jet printhead, is formed on an Si substrate, as shown in FIG. 14A.
The heaters are formed all at once as a thin film on, for example, a 6- or 8-inch Si substrate by, for example, sputtering using a CrSi alloy as a target in a gas mixture atmosphere containing nitrogen gas and argon gas. FIG. 14A shows an example in which the chips are formed on an Si substrate having a size of, for example, 6 inch or 8 inch. A plurality of heaters is built in each chip formed on the substrate. The resistance values of the heaters have an in-plane distribution on the 6- or 8-inch substrate. That is, the resistance values of the heaters are distributed even in a single substrate or a single chip in accordance with the location in the substrate or chip. The heater heating method of Japanese Patent Laid-Open No. H5-31899 stabilizes the heaters by simultaneously driving all heaters. If the variation between the heater resistance values depending on the positions on the Si substrate or the resistance value variation between the plurality of heaters built in a chip is large, it is impossible to execute fine process control according to the heater resistance value variation between the printing element arrays in the chip.